The strongly increasing proportion of power electronics in energy supply networks or mains, in particular in the field of drive engineering, means increasing distortion in the supply voltage due to the high harmonic contents of the current. In order to avoid harmonic currents in supply networks, standards specifying certain guidelines for manufacturers of electric and electronic devices have been issued in Europe over the last few years.
There are different active and passive solutions by different manufacturers worldwide for keeping to the standards, guidelines and recommendations issued. Depending on the power and application of the devices or the usage of the devices by the customer, these solutions may have advantages and/or disadvantages. Basically, active or passive devices and filters available at present for reducing current harmonics are not really attractive as to setup volume or cost and are thus only employed under certain circumstances.
For electronic devices having internal B2 and/or B6 rectifier circuits, the following conventional methods for reducing current harmonics are used: AC and DC chokes, higher-pulse rectifier circuits over B12, B18 or B24, acceptor circuit apparatuses, low-pass filters for 50 Hz or 60 Hz, special harmonic filters, means for an active sinusoidal current consumption (so-called active front ends) and active harmonic filters. The active harmonic filters here are operated in parallel on the network or mains.
Special harmonic filters will be explained in greater detail below. The special harmonic filters available at present exhibit a plurality of disadvantages, partly have very large setup volumes compared to consumers or cause immense expenses which often exceed the actual apparatus expenses of the consumer coupled thereto.
Since the circuit assembly of special harmonic filters fundamentally consists of inductive and capacitive components, three problems basically arise when operating the filter. High inductance values in the longitudinal branch of a filter result in a load-dependent voltage drops and may result in a reduced intermediate circuit voltage (direct voltage after a rectifier). This effect is partly compensated by connecting capacitances since capacitances raise the voltage again, however, a load-dependent voltage change will remain.
In addition, capacitors coupled in a shunt arm produce a capacitive reactive current which flows to the harmonic filter already under no-load conditions. A capacitive reactive current portion basically is to be kept very small since this so-called overcompensation is not desirable for energy supply companies. Some manufacturers of special harmonic filter thus offer the possibility of partly or completely switching off the capacitors under partial load conditions using a contactor. This in turn increases the expenses and complexity since such a contactor for a capacitive current should have suitable contacts and since the filter has to be integrated in the control flow.
Another disadvantage of conventional special harmonic filters to be mentioned is the resonance behavior of LC couplings. Basically, all circuits consisting of inductive and capacitive components have at least one resonance point. It is kept in mind in the case of filters that the frequencies arising are, if possible, not in the region of the resonance points, however, in dynamic load changes in connection with load changes at the supply network and/or switching on or off compensation units installed on the supply network, this is hardly foreseeable.
Thus, it shows that conventional special harmonic filters exhibit serious technological and economical disadvantages making usage thereof more difficult and/or expensive.